Reinforcing mineral binders of the cement or concrete type having glass fibers which are resistant to alkalines are known When such glass fibers are used, a problem of resistance and durability of the binder-fiber component arises. To eliminate this problem, one of the solutions contemplated is to add pozzolans to the cement matrix.
The term pozzolan is used here in the sense accepted by French Patent No. 2,149,998: they are silicates, of natural or synthetic origin, capable of reacting with lime and of being transformed into a hard and resistant material. They can be materials such as zeolites, illites or metakaolins for example.
Although the addition of pozzolan to cement reinforced with glass fibers is well known, all pozzolans do not confer on the final composite material the same mechanical properties. To select the pozzolans capable of conferring the best mechanical properties on the composite material, it is known to measure their specific surface area and their capacity for fixing lime. This is particularly the case with metakaolins.
The terms "metakaolin" and "metakaolinite" are used herein to mean an activated product of kaolinite, produced thermally or by any other means. The abbreviated formula for metakaolin can be written by using the standard symbols used by cement manufacturers, AS.sub.2 (A=Al.sub.2 O.sub.3 and S=SiO.sub.2). It is obtained, for example, by heat treatment of kaolinite at temperatures ranging between 700.degree. C. and 900.degree. C. for several hours.
French Patent No. 2,601,356 describes cement-based products containing glass fibers resistant to alkalines and containing, per 100 parts by weight of cement, from about 10 parts to about 40 parts by weight of metakaolin. The metakaolin entering into the composition of this product must exhibit a minimum reactivity measured according to the Chapelle test at 90.degree. C, as defined in the article by M. Benoit "Determination de l'activite pouzzolanique d'une pozzolane par voie chimique" [Determining the pozzolanic activity of a pozzolane, chemically]--Bulletin de liaison des Laboratories Routiers des Ponts et Chaussees No. 26, 1967, pages D1 to D5, complemented by the article by R. Largent "Estimation de l'activite pouzzolanique - recherche d'un essai" [Estimate of pozzolan activity--investigation of a test]--Bulletin de liaison des Laboratoires des Ponts et Chaussees No. 93 of January/February 1978, ref. 2143. The metakaolins thus selected make it possible to achieve products which exhibit, on the average, good mechanical properties. However, measurements made on such products show that all these metakaolins do not exhibit the same advantages in the long term.